Modulation meter



Feb. 25, 1941. G M, BROWN 2,233,196

MODULATION METER Filed May 31, 1939 CAPE/5E WA v5 cnvs/eeme Irwventor: George VI. Brown,

His Attorney.

Patented Feb. 25 1941 UNITED STATES MODULATION METER George M. Brown, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application May 31, 1939, Serial No. 276,586

11 Claims.

My invention relates to instruments responsive to predetermined maximum or minimum values of a varying voltage wave comprising an alternating component superimposed on a continuous component.

In the transmission of intelligence by modulated carrier waves, it is desirable to modulate the carrier to the greatest possible extent in order to make the most eificient use of it. When carrier waves are modulated by speech signals, instruments which indicate the average strength of the speech signals are of slight use in maintaining optimum conditions of modulation. This results from the fact that speech signals contain high peak values of very short duration. It-is desirable to use an instrument which will respond in the presence of these short high peaks. In the pulsating direct current normally used as the counterpart of speech, these 0 peaks are accompanied by similar valleys or points of minimum voltage. It is an object of my invention to provide an improved and simplified instrument which will be responsive to such maximum or minimum values of varying voltages 5 whenever those values respectively exceed or fall below a predetermined level.

It is common practice to provide for modulation of carrier waves by varying the voltage of the energy supplied to the carrier wave generator in accordance with the signal to be superimposed on the carrier. In a properly operated amplitude modulated transmitter with sine wave input it is possible to increase the modulating voltage until carrier cutoff occurs on negative peaks of modulation before overmodulation occurs. Any increase in modulating voltage beyond this point causes distortion because of flattening .of the carrier envelope on the zero voltage axis, rather than by extension of the maximum carrier amplitude beyond the normal level at twice carrier voltage.

In much music and most speech the wave formed by a microphone differs in amplitude on the two sides of the time axis. It is desirable in modulating a carrier with such waves that the side of the wave having the higher peaks shall produce the increases in the carrier envelope or the positive modulation peaks and-that the side of the waves having the lower peaks shall produce the minimum values of carrier amplitude or the negative modulation peaks.

In any case, it is clear that it is very desirable to limit the amplification of the modulating voltage in accordance with negative modulation peaks. Otherwise, if limiting be accomplished in accordance with maximum amplitudes of carrier, the higher peaks of modulating voltage may fortuitously be poled to produce minimum amplitudes of the carrier or negative modulation peaks which may result in serious radiation of undesired frequencies before the carrier has reached a maximum amplitude more than twice the unmodulated carrier level. Further, by poling the modulating voltage so that the higher peaks of the singal wave produce positive modulation peaks, the carrier is more efiiciently utilized. This follows since the energy radiated in a carrier having an envelopeof this type is greater than the energy radiated in a carrier having an envelope of inverse shape where the higher peaks of the signal wave produce minimum amplitude of carrier. This, of course, is true only if the amount of modulation is limited by the negative modulation peaks. It is therefore an object of my invention to provide an improved instrument responsive to predetermined instantaneous values of voltage on negative modulation peaks. It is also an object of my invention to provide a simplified and improved type of modulation indicator. 25

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing, in which the single figure represents schematically a portion.of a radio transmitter embodying my invention.

Referring to the drawing, an amplifier tube In is adapted to be energized from a suitable source of high frequency electrical waves (not shown). The cathode of the tube I0 is grounded and the anode is connected to one end of a tuned circuit including a tapped inductance II and a Variable condenser I2. An antenna [3 is connected to the tap of the inductance H. A suitable source of anode potential is connected through the secondary of an audio transformer M to the other end of the tuned circuit including the inductance ll. Variations in grid voltage in the tube I0 result in corresponding variations in anode voltage and in transfer of electrical waves through the tuned circuit to the antenna l3 from which radiation takes place.

A pair of tubes l5 and 16 are connected in push-pull relation and signal voltages are applied between the grids of, these two tubes from a suit- 55 able source (not shown) which may be a microphone and an audio amplifier, or the like. The anodes of the tubes l5 and It are connected, respectively, to the ends of the primary of the transformer I4. A suitable source of anode potential for these tubes is connected to the center tap of the primary of transformer Hi. When signal voltages are imposed on the grids of the tubes l5 and It, corresponding voltages are induced in transformer I4, which accordingly varies the anode voltage supplied to the radio frequency amplifier Iii. This variation in anode voltage results in a modulation of the ampli-,

tude of the high frequency wave generated by the tube [0.

Large amounts of distortion and corresponding radiation of undesirable frequencies which is called over-modulation occur when theanode voltage supply to tube it! becomes negative. It is desirable to adjust the signal voltage induced in transformer M to as great an average value as possible in order to make most efiicient use of the carrier Wave generated in tube H). The instrument, which I have invented, responds when the anode voltage of the tube 1 t falls below a certain predetermined value. This instrument includes a voltage divider connected at one end to ground and at the other end to a point between the tuned circuit including inductance H and the transformer M. This voltage divider comprises a resistor I1 and a potentiometer including an upper portion is and a lower portion l9. A condenser is connected across the ends of this voltage divider in order to prevent radio frequency voltages from appearing thereon. A resistor 2i and an energy storage device 22 in series are connected in shunt to the upper portion I8 of the potentiometer. While I have shown the energy storage device 22 as a condenser, any suitable energy storage device may be used. A grid controlled tube or relay 23, which is preferably of the gas or vapor filled type, has its cathode connected between portions 18 and I9, and hasits anode connected through a current responsive device 24 to the junction between resistor 2| and energy storage device 22. This current responsive device 24 maybe a visual or audible indicating device, orit may be suitable means for adjusting the averagelevel of voltage induced in transformer 14. A sliding contact on the portion E9 of the potentiometer is connected to a resistor 25, the other end of which is connected to the grid of tube 23. An energy storage device 26 connects the grid of tube 23 to a second sliding contact which may be adjusted over both portions 18 and. l9 of the potentiometer. It is preferably adjusted only on the lower portion IQ of the potentiometer, as will be more fully explained hereafter. While Ihave shown the energy storage device 26 as a condenser, any suitable energy storage device may be used.

In operation, the energy storage device 22 becomes charged to a potential which is the average potential existing across portion l8 of the potentiometer. Resistor 2| is of sufficient size that the energy storage device or condenser 22'does not discharge substantially during the average time of variation in the signal voltage from transformer M. The voltage across the energy storage device or condenser 22 therefore maintains the anode of tube 23 at a considerable positive potential with respect to its cathode. This potential is a function of the-average anode voltage supply to. tube 10. Any other suitable source of operating potential may be connected from the cathode of tube 23 through the device 24 to the anode of tube 23. However, it is preferred that this operating potential be provided by connecting a reactance such as condenser 22, as shown.

With adjustments as shown, upon the occurrence of a minimum voltage across the voltage divider such that tube 23 breaks down, energy storage device 22 discharges through the tube or discharge device 23 and thus removes anode operating potential from it before the voltage on the voltage divider again rises. The grid of the device may then regain control of the anode current'before anode voltage is reapplied. This is assured by resistance 2| which delays the charging of energy storage device 22 and thus delays the application of anode voltage to the discharge device 23. Thus by the use of energy storage device 22 to supply operating potential to'tube 23, an indication isobtained at each 00- currence of abnormal voltage and upon the cessation of such abnormal voltages the indications cease.

The energy storage device 26 is charged to a potential corresponding to the average potential which exists between its sliding contact and the sliding contact of resistor 25. This resistor- 25 is of sufficient value to prevent discharge of energy storage device 26 in the average time,

of signal voltage variations on transformer 14-. It is evident that the grid of tube 23 is maintained at an average negative potential with respect to its cathode. This negative bias poten-' tial is measured by the average value of potential from the sliding contact of resistor 25 to the junction between portions l8 and H] of the potentiometer. Alternatively viewed, the energy storage device 26 maintains the grid of tube 23 at siderable average negative bias be applied to the grid by adjustment of the sliding contact of resistance 25, the value of positive or negative modulationpeak voltages; at which an indication may be obtained, may be adjusted by adjustment of the sliding contact connected to energy stor: age device 26. 'If this latter sliding contact be adjusted over portion IQ of the potentiometer, negative modulation peaks are indicated. If it be adjusted over portion 3, positive modulation peaks are indicated. As explained above, it is preferred that the indications be madeof negative modulation peaks rather than positive, and hence that adjustment of the sliding contact be made only over portion IS; The amount of adjustment of this sliding contact regulatesthe modulation depth at which an indicationisobtained. be made a fixed connection and the adjustment of the other sliding contact be used; tomakethe instrument. indicate. at various predetermined values of modulation peak voltages.

. Although I prefer to. use a circuit. including a reactance such as condenser 26, it ispossible to construct. an indicator such as tissh'own without such. an energy. storage-device. If the tube-23 Obviously, either sliding contact; may

of 100 per cent. If, however, the tube 23 be of the type which allows anode current to flow when the grid becomes less negative than a certain negative potential, negative peaks of modulation are-indicated for modulation depths of less than 100 per cent. The modulation at which indication is obtained is adjustable by movement of the sliding contact connected to resistor ,25.

Movement of this contact varies the ratio between the average biasing potential supplied to tube 23 and the negative potential at which the grid allows tub 23 to pass current. By varying this ratio the amount of alternating voltage necessary to raise the grid to a potential at which the present application. It is preferred to use condenser 26 since the relative amounts of average biasing potential and alternating potential applied to the grid of tube 23 may be separately preadjusted.

The instrument may be adjusted for use with any value of varying voltage by making the resistor I! of suitable size. If desired two or more of these instruments may be used on a single transmitter. One may be adjusted to indicate at peaks of a certain value, for example, '75 per cent modulation, and another may be adjusted to operate upon peaks of different value, for

example, per cent modulation. I prefer, as I explained above, that both instruments operate on negative peaks of modulation. The adjustment of the voltage induced in transformer [4 may then be made such that the instrument operating on 75 per cent modulation responds occasionally while the instrument which operates on 951 per cent modulation very rarely responds.

It is evident that this instrument including the portions I8 and I9 of the potentiometer may be'used to indicate maximum or minimum peak values'of voltage between any two points.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed, and I aim by the appended claims to cover any such modifications as fall within the respect to said cathode as to prevent current in said anode, and means to maintain said grid at a fixed potential with respect to a third point on said source, said third point being removed in potential from said cathode to such an extent that, upon predetermined variation in potential between said points, said grid allows current to flow in said anode and said first means is actuated.

2. In combination, a source of variable voltage, an electron discharge device having an anode, a cathode, and a grid, a current responsive device connected to said anode and cathode,

-means to impress a voltage between said cathode and anode wherebysaid current responsive device responds upon current flow therethrough, said cathode and grid being connected to points on said source, the point to which the grid is connected being at a potential with respect to said cathode svfiicient to prevent flow of current in said anode, and means to maintain said grid at fixed positive potential with respect to a third more negative point on said source during variationsin voltage of said source, whereby during such variations said cathode assumes a potential with respect to said grid such that current flow in said anode is initiated, and said current responsive device is actuated.

3'. In combination, a source of variable voltage, an electron discharge device having an anode, a cathode, and a grid, means including a current responsive device connected to said anode and cathode for impressing voltage therebetween, said cathode and grid being connected to points on said source, the point to which the grid is connected being at a potential with respect to said cathode sufficient to prevent flow of current in said anode, and means to maintain said grid at fixed negative potential with respect to a third more positive point on said source during variation in voltage of said source, whereby during such variation said cathode assumes a potential with respect to said grid such that cur rent flow in said anode is initiated, and said current responsive device is actuated.

4. In combination, a source of variable voltage, an electron discharge device having an anode, a cathode, and a grid, a current responsive device connected to said anode and cathode, means for impressing a voltage between said anode and cathode whereby said current responsive device responds to current flow thereof current in said anode, a condenser connected between said grid and a third more positive point on said source, and means to maintain the potential on said condenser substantially constant during variations in voltage of said source, whereby during such variations said cathode assumes a potential with respect to said grid such that current flow in said anode is initiated, and said current responsive device is actuated.

5. In combination, a source of variable voltage, an electron discharge device having an anode, a cathode, and a grid, a current responsive device connected to said anode and cathode, means for impressing a voltage between said anode and cathode whereby said current responsive device responds to current flow therebetween, said cathode and grid being connected to points on said source, the point to which the grid is connected being at a potential with respect to said cathode suflicient to prevent flow of current in said anode, a condenser connected between said grid and a third more negative point on said source, and means to maintain the potential on said condenser substantially constant by during such variations said cathode assumes connected to a point on said source, means connecting an energy storage device between said point and a second point on said source of more positive potential whereby energy is stored in said storage device, said anode being connected to said second point, a current responsive device in circuit with said cathode and anode, means for applying to said grid a voltage with respect to said cathode sufiicient to prevent initiation of current in said anode, and means to maintain said grid at a fixed potential with respect to a third point on said source, said third point being removed in potential from said cathode to such an extent that, upon predetermined variations in potential between said points, said grid allows initiation of current in said anode, and said current responsive device is actuated by current from said storage device.

7. In combination, a source of variable voltage, an electron discharge device having a grid, a cathode, and an anode, said device having the characteristic that no current is initiated in said anode except when the grid is more positive with respect to said cathode than a predetermined fixed potential, said cathode being connected to a point on said source, means connecting an energy storage between said point and a second point on said source of more positive potential, whereby energy is stored in said storage device, said anode being connected to said second point, a current responsive device in circuit with said cathode and anode, means for applying to saidgrid a voltage with respect tosaid cathode sufiicient to prevent initiation of current in said anode, and means to maintain said grid at a fixed potential with respect to a third point on said source, said third point being removed in potential from said cathode to such an extent that, upon predetermined variations in potentials between said points, said grid allows initiation of current in said anode, and said current responsive device is actuated by current from said storage device.

8. In combination, a voltage divider adapted to be connected to a source of variable voltage, a relay having a main circuit and a control circuit for controlling the current in said main circuit, a source of potential and a current responsive device in said main circuit, the terminals of said control circuit being connected to points on said voltage divider, one of said terminals being connected to a point of such potential with respect to the other as to prevent current flow in said main circuit, and means to maintain said one of said terminals at a fixed potential with respect to a third point on said voltage divider, said third point being removed in potential from the other of said terminals to such an extent that, upon predetermined variations in potential between said points, said current responsive device is actuated.

circuit, means connecting an energy storage device to receive energy from said voltage divider, a current responsive device'connected in series with said energy storage device in said main circuit, means to impress a voltage on said control circuit of such value as to prevent current flow in said main circuit, vand means connecting said. control circuit to said voltage divider to impress a different voltage on said control circuit of such value that, upon predetermined variations in potential on said voltage divider, said current responsive device is actuated by current arising from discharge of said energy storage device.

10. The combination with a carrier Wave generator having a source of potential, and meansfor varying said potential in response to signals to modulate the amplitude of said wave, of means for indicating the depth of said modulation comprising an electron discharge device having a grid, a cathode, and an'anode, means including a current responsive device connected .to said anode and cathode for impressing a voltage therebetween, a voltage divider, means for impressing said varying potential across said voltage divider, said cathode being connected to a point on said divider, means for applying to said grid a voltage from said divider sufficient to prevent initiation of current in said-anode, and means to maintain said grid at a fixed potential with respect to a second point on said "divider, said second point being removed in potential from said cathode to such an extent that, upon predetermined variation in potential between said points, said grid allows current to flow in said anode and said current responsive device is actuated.

11. The combination with a carrier wave generator having a source of potential, and means for varying said potential in response to signals to modulate the amplitude of said wave,

of means for indicating the depth of saidmodulation comprising a vapor electric discharge de- .vice having a cathode, an anode, and a control member, an energy storage device and a current responsive device connected to said cathode and anode, a voltage divider, means for impressing said varying potential across said voltage divider, said cathode being connected to a point :on said divider, -means for storing electric energy from said divider in said energy storage device,--means for applying to said control mem ber a voltage with respect to said cathode suffis cient to prevent initiation of current in said anode, and means to maintain said control member at a fixed potential with respect to a second point on said divider, said second point being removed in potential from said cathode to such an extent that, upon predetermined variation in potential between said points, said control member allows initiation of current in said anode and said current responsive device is actuated by current from said storage device.

GEORGE M. BROWN. I 

